Multiphoton vibrational–rotational transitions in diatomic molecules

An analysis is made of multiphoton vibrational–rotational transitions in diatomic molecules which take place on exposure to radiation having a frequency close to that of the vibrational quantum. Expressions are obtained for the multiphoton matrix elements and dynamic Stark shifts of the resonance frequencies. It is found that in the case of large rotational momenta $J\gg1$, the dependence of the multiphoton matrix elements and transition probabilities on the polarization of the radiation and on the projection of the rotational momentum is governed by a universal rotational factor. The probability of transitions with $\vert\Delta J\vert<\Delta v$ decreases rapidly as $J$ increases, so that transitions with $\Delta J = \pm\Delta v$ are the most probable. Some characteristics of optical nutation in molecular gases are studied. A characteristic feature of multiphoton transitions is the possibility of observing undamped nutations. In some cases, the response of the system resembles random noise.